Potentially dangerous situations can occur when hoses carrying compressed air become disconnected, because the end of the hose tends to flail about as a result of the compressed air blowing off. To prevent this, most compressed air systems or tools incorporate a safety valve. This often used to be a simple one-way valve. However, two-way safety valves are also known. For example British Pat. No. 2139326 discloses a two-way safety valve which has the advantage of being able to operate equally well with flows in either direction, as well as considerably facilitating tool change overs. Valves of this type can be provided in standard sizes according to the bore of the air line. Different applications, however, will demand different air pressures and flow conditions under which the valve must operate. The extent of the difference in possible pressures and flows is surprisingly large, particularly for smaller bore sizes, which necessitates the provision of a wide range of standard valves. In recognition of this problem, I have not developed an improved valve.